1. Field of the Invention
The present invention generally relates to pressure indicators, and more particularly to a simple device which allows quick visual indication of whether the monitored fluid pressure is less than, equal to or greater than a predetermined pressure.
2. Description of the Prior Art
In many applications which employ pressure gages, there is no need for the user to know the exact pressure, but only whether it is above or below a certain desired pressure. There have been many pressure indicating devices proposed for presenting a visual indication at a certain preselected fluid pressure, as alternatives to the traditional Bourdon tube pressure gage. The history of inventions in this field in the patent literature attests to a long-felt need for a practical and inexpensive pressure indicator. However, these prior devices have not met with commercial success because they have in general been overly complex, and therefore expensive to manufacture, or have been characterized by having a gradual, low resolution response to pressure, presenting an indication which is indefinite and difficult to interpret.
Among the prior art, U.S. Pat. No. 3,703,879 (Huthsing) teaches a pressure indicator comprising a housing, window pane means, an elastomeric diaphragm moving under pressure towards the window pane means to operate an indicating means. The first of two embodiments of indicating means disclosed by Huthsing includes an indicating fluid, which gradually moves up an indicator window when displaced by the deformed pressurized diaphragm, and the other embodiment having a raised portion on the diaphragm which moves under pressure towards an inside frosted viewing window. The fluid indication depends on gravity, and therefore suffers degradation of the indication in certain orientations. Both indicating means suffer from vague and imprecise indications, in that large displacements of the diaphragm produce only minor visual effects in the viewing window.
U.S. Pat. No. 3,987,699 (Popenoe) teaches an opti-mechanical displacement microindicator comprising a transparent flexible envelope containing a light-absorbing fluid, a viewing window and an opposite wall indicator portion. This device is characterized by extreme simplicity and sensitivity, changing its appearance dramatically, for instance, from black to bright red when deformed by as little as 0.001 inch. The instant invention uses features of this microindicator in a pressure-indicating device.
U.S. Pat. No. 5,014,786 (Kobayashi) teaches a pressure checker comprising a main unit having a hollow bore, a spool acted upon by the monitored pressure, a compression spring biasing the spool in opposition to the pressure; whereby the spool under pressure is displaced axially to press upon a microdisplacement indicator as taught in U.S. Pat. No. 3,987,699 (Popenoe), thus providing visual indication of the desired pressure. This device will have a high sensitivity, in that a small pressure change will result in a dramatic visual color change as viewed through the window, but suffers from unnessary complexity, as well as considerable friction in the internal parts leading to insensitivity and excessive deadband.
The prior art devices have not provided a simple, inexpensive and compact visual pressure indicator offering high sensitivity and rapid change of indicator presentation over a narrow pressure range. Clearly, many applications which presently use Bourdon tube gages could benefit from substitution of a simple and practical go/no go type visual pressure indicator.